In filling containers by means of a funnel, it is often difficult to determine when to stop pouring without causing overflowing. This is particularly true in cases where the container being filled has opaque walls, such as a gasoline tank or lantern. In the past, funnels have been provided having gaskets for sealing the filling opening of containers and with vents which permit displaced air in the container to escape to atmosphere until the liquid level closes the vent. Normally, these prior art devices have not included signalling apparatus or have included an audible signal that required substantial compression of the displaced air before the signal could be heard.
Other prior art devices have used a stream of bubbles as a signal; however, these devices have been for use with containers which were open to atmosphere. Some examples of the prior art are the U.S. patents to Balden No. 657,080; Barney No. 918,814; Mapel No. 1,672,983; Rektorys No. 2,871,895; British Pat. No. 7,959 of 1896 to Robinson; and French Pat. No. 564,228 of 1923 to Hutchison.